LSD MDMA Psilocybin In Der Psychotherapie STUDIEN

Title page

Formatted for Psychological Medicine (original report)

The paradoxical psychological effects of LSD

Carhart-Harris RL 1 *, KaelenM 1 , Bolstridge M 1 , Williams TM 3 , Williams LT 1 , Feilding
A 2 , Nutt D J 1

1 Imperial College London, Centre for Neuropsychopharmacology, Division of Brain
Sciences, Faculty of Medicine, London, UK

2 The Beckley Foundation, Beckley Park, Oxford, UK

3 The University of Bristol, Department of Psychiatry, Bristol, UK
*r.carhart-harris@ imperial.ac.uk

Tel: 02075942679
Fax: 02075946548

Acknowledgements : This research received financial and intellectual support from the
Beckley Foundation and was conducted as part of a wider Beckley- Imperial research
programme. The researchers would also like to thank supporters of the Walacea.com crowd-
funding campaign who played a crucial role in securing the completion of this study. The
report presents independent research carried out at the NIHR/Wellcome Trust Imperial
Clinical Research Facility.

1

Abstract (Limit = 250 words, currently 242 words)

Background: Lysergic acid diethylamide (LSD) is a potent serotonergic hallucinogen or
psychedelic that modulates consciousness in a marked and novel way. This study sought to
examine the acute and mid-term psychological effects of LSD in a controlled study.

Methods: Twenty healthy volunteers participated in this within- subjects study. Participants
received LSD (75pg, Lv.) on one occasion and placebo (saline i.v.) on another, in a balanced
order, with at least 2 weeks separating sessions. Acute subjective effects were measured
using the Altered States of Consciousness (ASC) questionnaire and the Psychotomimetic
States Inventory (PSI). A measure of optimism (the Life Orientation Test, LOT-R), the NEO
PI-R personality inventory, and the Peter’s Delusions Inventory (PDI) were issued at baseline
and 2 weeks after each session.

Results: LSD produced robust psychological effects; including heightened mood but also
high scores on the PSI, an index of psychosis-like symptoms. Increased optimism and trait
openness were observed two weeks after LSD (and not placebo) and there were no changes in
delusional thinking.

Conclusions: The present findings reinforce the view that psychedelics elicit psychosis-like
symptoms acutely yet improve psychological wellbeing in the mid to long-term It is
proposed that acute alterations in mood are secondary to a more fundamental modulation in
the regularity of cognition. Moreover, it is proposed that increased cognitive flexibility
subsequent to 5-HT2 aR stimulation, promotes emotional lability during intoxication and
leaves a residue of ‘loosened cognition’ in the mid to long-term that is conducive to improved
psychological wellbeing.

2

(Main article word limit = 4500, currently 4800, without subtracting citations in body of
text)

Introduction

Lysergic acid diethylamide (LSD) is a potent serotonergic hallucinogen or ‘psychedelic’ that
alters consciousness in a marked and unusual way. Hie dmg was first intentionally consumed
by the Swiss chemist Albert Hofmann in 1943 in a self- experiment in which he ingested
250pg (a high dose) in his laboratory before travelling home. In a detailed report of his
experience, written a few days later, Hofmann describes an initially unpleasant experience,
characterised by altered perception, fear and paranoia: his next-door neighbour transformed
into a “malevolent, insidious witch with a coloured mask”, he sensed a “disintegration of the
outer world”, a “dissolution of [his] ego” and was “seized by a dreadfiil fear of going insane”
(Hofmann, 1980).

From this account, it would be reasonable to suspect that Dr Hofmann was negatively
affected by this experience but his description of his mental state the next day suggests
otherwise: “I then slept, to awake the next morning with clear head... A sensation of
wellbeing and renewed life flowed through me. Breakfast tasted delicious and gave me
extraordinary pleasure. When I later walked out into the garden, in which the sun shone now
after a spring rain, everything glistened and sparkled in a fresh light. The world was as if
newly created. ’’(Hofmann, 1980)

When LSD was first distributed by Sandoz pharmaceuticals inl948, product guidelines
stipulated two main applications: 1) analytical psychotherapy and 2) experimental studies on
psychoses. The rationale for the former was that LSD could “elicit [the] release of repressed
material and provide mental relaxation for anxiety and obsessional neuroses”, and for the
latter, that it could model aspects of psychosis and facilitate an understanding of its nature
and pathogenesis (Hofmann, 1980). These two properties formed the basis of a large number
of research projects with LSD in the 1950s and 60s. However, the apparent paradox by which
the same compound can be both a model of, and yet a treatment for psychopathology, has
never been properly addressed.

3

In the early years of research with LSD, its remarkable potency (LSD is psychoactive in
doses of 25pg or lower (Hintzen and Passie, 2010) ) led psychiatrists to speculate about the
existence of an endogenous LSD- like ‘schizotoxin’ in the brains of patients with
schizophrenia (Osmond and Smythies, 1952). In subsequent years however, focus shifted
more onto therapeutic applications, such as treating alcohol dependence, mood disorders and
anxiety related to dying. Human research with LSD was brought to a halt in the late 1960s
due to political pressure, motivated in part by reports of adverse psychological reactions
among people using the dmg improperly. Ironically however, at the same time, reports of
therapeutic success in the treatment of various psychiatric disorders were beginning to
amount (Grinspoon and Bakalar, 1979). In the last 18 months, five new reports on clinical
research with LSD have appeared in scientific press (Gasser et aL, 2014, Carhart- Harris,
2014, Schmid et aL, 2014, Dolder et aL, 2015, Kaelen et al., 2015), one of which focused on
the drug’s therapeutic effects (Gasser et aL, 2014) and another, its psychotomimetic (Schmid
et aL, 2014).

Clinical research with psychedelics is currently undergoing a major revival and modem
studies are documenting the same paradoxical properties that were historically described with
LSD. For example, in a controlled study in healthy volunteers, high dose psilocybin produced
strong or extreme fear in 30% of healthy volunteers and yet 80% reported improvements in
wellbeing after the experience, with none reporting any decreases (Griffiths et al, 2006).
Remarkably, in follow-up of the same sample, 65% reported improved wellbeing 14 months
after their (single) psilocybin experience (Griffiths et aL, 2008) and significant increases in
the personality trait openness were also evident (MacLean etaL, 2011). These finding in
healthy volunteers are supplemented by an increasing number of patient studies. Clinical
improvements have been observed with psilocybin-assisted psychotherapy for the treatment
of tobacco (Johnson et al., 2014) and alcohol addiction (Bogenschutz et aL, 2015), obsessive
compulsive disorder (Moreno et al., 2006) and anxiety related to dying (Grob etaL, 2011).
Many of these reports mention some psychological discomfort during the acute experience;
yet, the therapeutic benefits have been impressive and enduring.

4

Case reports of persistent psychological problems apparently precipitated by a psychedelic
have considerable potential to alarm and excite concern (Reich and Hepps, 1972). However,
such cases are rare and largely restricted to recreational use. Evidence does not support the
view that psychedelics are harmful to mental health (Hendricks et al, 2015). Indeed, to the
contrary, two recent population studies found decreased rates of suicidality and psychological
distress among persons reporting previous use of psychedelics (Hendricks etal, 2015) and no
evidence of any increased rates of mental health problems (Krebs and Johansen, 2013).
Similarly, large meta- analyses of controlled research have found that cases of mental health
complications following exposure to a psychedelic are extremely rare (i.e. < 0.1%), even in
vulnerable populations (i.e. < 0.2%), and are rarer still if volunteers are properly screened
(Cohen and Ditman, 1962, Studems et al., 2011).

The main aim of the present study was to investigate the acute and ‘mid-temT (ie. 2 weeks
after the acute experience) psychological effects of LSD in a placebo-controlled study in
healthy volunteers. Validated measures of personality, optimism and psychotic symptoms
were collected at baseline and two weeks post LSD/placebo and measures of mood, cognition
and psychotomimetic states were collected at the end of each dosing day. It was predicted
that LSD would induce emotional lability and psychosis-like symptoms acutely but increase
psychological wellbeing and openness in the longer term.

Methods

Experimental design

This was a placebo- controlled, within-subjects/cross-over study, with a balanced order
design. Twenty healthy volunteers were recruited to the study via word of mouth. The study
received a favourable opinion from NRES committee London - West London and was
conducted in accordance with Good Clinical Practice guidelines, NHS Research Governance
Lramework and complied with the ethical standards of the declaration of Helsinki (1975,
revised 2008). Imperial College London sponsored the research and a Home Office license
was obtained for research with schedule one drugs. All volunteers were sent a study
information sheet and asked to read it before their screening visit.

5

Volunteers made three study visits: screening, dosing session one and dosing session two.
Dosing sessions were separated by at least two weeks in every case and the order of receipt of
LSD was balanced, re. half of the volunteers received LSD in dosing session one and half in
dosing session two. Volunteers were blind to the dosing order but the research team were not.
Dosing order was determined sequentially so that all odd numbered volunteers (re., SI was
the first volunteer recruited and S20 was the last) received LSD in dosing session one,
whereas all even numbered volunteers received it in dosing session two. Subjective ratings
were completed electronically and remotely soon after screening (baseline) and two weeks
after a dosing session and sent to the researchers via email. Volunteers were instructed to
complete the questionnaires in a quiet space without rushing. Two questionnaires pertaining
to the acute drug experience were completed electronically within the research centre at the
close of dosing days.

Screening

Prior to study enrolment, volunteers attended a screening visit at the Imperial Clinical
Research Facility (ICRF) at the Hammersmith hospital in West London. The study design,
procedures and psychological effects of LSD were explained and signed informed consent
taken. Key exclusion criteria were: <21 years of age, personal history of diagnosed
psychiatric illness, immediate family history of a psychotic disorder, an absence of previous
experience with a classic psychedelic drug (e.g. LSD, mescaline, psilocybin/magic
mushrooms or dimethyltryptamine/ayahuasca), pregnancy, problematic alcohol use (assessed
via psychiatric interview and reported weekly units), or a medically significant condition
rendering the volunteer unsuitable for the study. Hie decision to recruit only individuals with
prior experience with psychedelics was motivated by safety considerations, i.e. to min im ise
the risk of an adverse response to the drug. Screening involved routine blood tests,
electrocardiogram, heart rate, blood pressure, and a brief neurologic examination. All
enrolled participants were deemed physically and mentally healthy by the study psychiatrist,
and none had histories of drug or alcohol dependence or diagnosed psychiatric disorder.
Volunteers were asked to remain abstinent from alcohol the evening before a dosing day and
to refrain from using other recreational dmgs for the duration of the study.

6

Study procedures

Participants were asked to arrive at the study centre (Cardiff University’s Brain Research
Imaging Centre, CUBRIC) at a specific time at or before 9:00am. A urine test for dmgs of
abuse and pregnancy (where relevant) was carried out. Participants were re-briefed about the
procedures for the day and any recent drug and alcohol use was documented. The study
physician inserted a cannula into a vein in the antecubital fossa in preparation for intravenous
dosing and the volunteer was encouraged to relax prior to drug/placebo administration. The
dose of LSD was 75pcg (p.o.) in 10ml saline. Previous research has found this dose to
produce robust psychological effects that are generally well tolerated (Carhart-Harris, 2014).
Placebo was 10ml saline (i.v.). Both solutions were infused over 2 minutes.

After dosing, volunteers completed a functional neuroimaging protocoL This aspect of the
study will be covered in detail in a separate publication. In brief, subjects spent a period
habituating to a scanner environment in a mock- magnetic resonance imaging (MRI) scanner
before entering a real MRI scanner (one hour post-dosing). The MRI scanning session lasted
for one hour after which a structured interview was performed and some ratings completed.
Participants entered a magnetoencephalography (MEG) scanner approximately 3.5 hours post
dosing. MEG scanning lasted for just over one hour. Participants were then interviewed and
completed a battery of cognitive and behavioural tests (these will be detailed in a separate
publication). Tasks were completed 6-7 hours post dosing.

The subjective effects of LSD were detected approximately 10 minutes post- infusion, peaked
approximately 120 minutes post- infusion, and subsided to a negligible level approximately 7-
8 hours post infusion. Participants were discharged by the study physician when they were
considered to be functioning normally. Volunteers were either picked- up by a friend or
partner, ordered a taxi, or accompanied home by the research team as far as was feasible.
Volunteers were asked to contact a researcher via phone or text message once they had
arrived home safely. A study psychiatrist was present for the duration of each dosing session

7

and one researcher was allocated to each participant to assist them throughout the day. For
each participant, the same researcher was present for both dosing days.

Main outcomes and measures
Acute outcomes

Participants completed two questionnaires at the end of each study day before being
discharged by the study physician. These questionnaires enquired about different aspects of
the subjective experience. The first was the altered states of consciousness questionnaire
(ASC); a well validated and widely used tool for defining different altered states of
consciousness that has been usefully revised in recent years (Studems et aL, 2010) and the
second was the psychotomimetic states inventory (PSI), a tool developed to probe the
psychotomimetic effects of different drugs (Mason et aL, 2008). Participants were asked to
complete the questionnaires with reference to the peak subjective effects of LSD (Le. when
the effects were most intense) or with reference to how they generally felt throughout the day
- e.g., if they did not notice any effects.

Mid-term outcomes

Mid-term outcomes were completed at baseline and two weeks after each dosing session.
These included: the revised life orientation test (LOT-R) (Glaesmer et aL, 2012), the NEO PI-
R personality inventory (Costa and McCrae, 1995) and Peter’s delusions inventory (PDI)
(Peters et aL, 1999). The LOT-R was chosen as it is a well- validated measure of trait or
dispositional optimism (Scheier et aL, 1994), the NEO PI-R was chosen as it is well- validated
and previous research has shown its sensitivity to the enduring effects of psychedelics
(MacLean et al., 2011), and the PDI was chosen as it is a well- validated measure of
delusional thinking that has shown to be sensitive to psychotic-like symptoms in the general
population (Peters etaL, 1999).

Additional measures

8

Additional questionnaires completed at baseline included the Beck depression inventory
(BDI) (Beck et aL, 1961), the quick inventory of depressive symptoms (QIDS) (Rush et aL,
2003), the state-trait anxiety inventory (STAI) (Hodgues and Spielberger, 1969), the
ruminative response scale (RRS) (Roelofs et aL, 2006), the dysfunctional attitudes scale
(DAS) (Floyd et aL, 2004) and the modified version of the Tellegen Absorption Scale
(MODTAS) (Tellegen and Atkinson, 1974).

Data analysis

Repeated measures ANOVAs were used to test between- condition differences in the ASC,
PSI and PDI, which have multiple factors. However, since a strong prior hypothesis was held
that the personality trait ‘openness’ would be significantly increased post-LSD (MacLean et
aL, 2011), this was analysed using a paired t-test rather than an ANOVA. The remaining four
personality dimensions in the NEO PI-R were analysed using paired t-tests with Bonferonni
correction for multiple comparisons. A paired t-test was used to analyse changes in the LOT-
R as this has only one dimension. To test for order confounds in relation to primary
outcomes, order was included as a variable in significant tests, and to further test for the
influence of placebo/participation, baseline versus 2 weeks post-placebo differences in
outcomes were tested for those who received placebo in their first dosing session (potential
carry over effects of LSD precludes the inclusion of those who received placebo in their
second dosing session). Effect sizes (Cohen’s d) were calculated for primary outcomes. For
correlational analyses, Pearson product- moment coefficients were calculated and 2-tailed
hypotheses tested. Hie problem of multiple comparisons was accounted for by Bonferonni
correction.

Results

Participants

Twenty healthy volunteers participated in the study (4 females, mean age = 30.9 ± 7.8, range
= 22-47). All had at least one previous experience with a classic psychedelic drug (mean
estimated LSD uses = 14 ± 17.8, range = 0-70) but not within 14 days of the first dosing

9

session (mean last use of LSD = 899.3 ± 1363, range = 14-5400 days). Self- estimates of other
drug-use were as follows (mean, SD, range): weekly alcohol units = 10.3 ± 9; daily cigarettes
= 0.3 ± 1.1, 0-5; cannabis uses = 705 ± 639, 30-2000; MDMA uses =21 ± 24, 2-100;
psilocybin/magic mushroom uses = 9.4 ± 7.8, 1-35; ketamine uses = 3.6 ± 5, 0-20; cocaine
uses = 9.6 ± 9.4, 0-30. Other baseline scores were as follows: BDI = 0.6 ± 0.9, 0-3; QIDS =
4.2 ±3.1, 0-10; DAS = 105.5 ±23, 55-146; RRS = 35.4 ± 7.9, 24-52; STAI = 29.2 ± 4.6, 20-
38; MODTAS = 58.3 ± 19.8, 19-89.

Acute subjective effects of LSD

A repeated measures ANOVA revealed a significant main effect of dmg (F[l,19] = 82.6, p <
0.001) and a significant drug x ASC interaction (F[10, 190] = 10.3, p < 0.001). Post-hoc t-
tests revealed that all 11 dimensions or factors of the ASC were rated significantly higher
after LSD than placebo (p < 0.01). Mean scores for each factor are displayed in the radar
chart in figure 1. It is notable that although the LSD experience was dominated by changes in
visual perception (i.e. elementary imagery, complex imagery and audio-visual synaesthesia),
the factor ‘blissful state’ was markedly elevated under the drug (±0.37 ± 0.28, Cohen’s d =
1.65). Although still significantly increased, the factor ‘anxiety’ was the least elevated under
LSD (±0.15 ±0.2, Cohen’s d= 1.03). These results indicate a marked increase in emotional
arousal and lability under LSD but with a distinct bias towards positive affect. Reinforcing
this, a t-test revealed that increases in ‘blissful state’ under LSD were significantly greater
than increases in ‘anxiety’ (t = -3.7, df = 19, p < 0.01, Cohen’s d = 0.91).

10

Altered states of consciousness (ASC) questionnaire

Experience of unity

— Placebo
—LSD

Audio/visual

synaesthesia

Meaning

09

0.6

07

0 b
0.5
0.4

Spiritual experience

Blissful state

Elementary imagery

Insightfulness

Complex imagery

Disembodiment

Anxiety

Impaired cognition

Insert figure 1 here (ASC chart)

Figure 1. Acute effects of LSD measured via the ASC: Displayed are the mean scores on
each of the 11 dimensions of the ASC for the LSD (black) and placebo (gray) conditions.

A repeated measures ANOVA revealed a significant main effect of condition on the PSI
(F[l,19] = 65.19, p < 0.001) and a significant condition x PSI interaction (F[5,95] = 35.2, p <
0.001). Mean scores for each factor of the PSI are shown in the bar chart in figure 2. Post-hoc
t-tests revealed that all factors, except for ‘anhedonia’, were significantly increased under the
drug (p < 0.01). The most marked effect was on ‘cognitive disorganisation’ (Cohen’s d =
2.37) which is related to thought-disorder in psychosis, but other highly characteristic aspects
of psychosis such as ‘delusional thinking’ (Cohen’s d= 1.63) and ‘paranoia’ (Cohen’s d =
0.83) were also markedly increased under the drug.

11

Psychotomimetic states inventory (PSI)

60

50

Delusional Perceptual Cognitive Anhedonia Mania Paranoia Total score

thinking distortion disorganisation

Insert figure 2 here (PSI chart)

Figure 2. Acute effects measured of LSD measured via the PSI. Displayed are the mean
scores plus the positive standard errors from the mean (SEM) for each of the 6 factors of the
PSI, plus the total score (which is a sum of the scores of the 6 factors). AH factors, except for
anhedonia were scored significantly higher under LSD than placebo.

Mid-term subjective effects of LSD

Optimism was significantly increased two weeks after LSD (t = 2.91, df = 18, p = 0.005,
Cohen’s d = 0.56), as was trait openness (t = 1.95, df = 19, p = 0.03, Cohen’s d = 0.16)
(figure 3), and there were no such changes in optimism or personality post-placebo. In
exploratory t-tests, there was a trend towards an increase in trait agreeableness post-LSD (t
=2.2, df = 19, p = 0.038, Cohen’s d = 0.21); however, this did not survive correction for
multiple comparisons (corrected p = (0.038 x 4) = 0.15).

12

Optimism (IOT-R)

iso

* 0 . 00 $
n- 20

ISO

140

X

3 100

«0

40

Bsieline

JO

0

2 we«*5 post - ISO

Openness (NEO PI*R)
t ;

.! i

ftjsHmt 2 weeks post-LSD

Insert figure 3 here (combined bar charts)

Figure 3. Mid-term effects of LSD: These charts display paired data- points for optimism
(chart on the left) and openness (chart on the right) scores at baseline and 2 weeks after LSD
for each participant. Also displayed on each chart are the mean scores (horizontal bars) and
the positive SEM. Both optimism and openness were significantly increased after LSD. There
was no effect of order of receipt of LSD nor were there any changes in optimism or
personality 2 weeks after receipt of placebo.

Repeated measures ANOVA found no change in delusional thinking (PDI scores) 2 weeks
after LSD (p > 0.05). In fact, exploratory t-tests suggested a trend towards less distress (t =
1.92, df = 19, p = 0.068) and preoccupation with ‘delusional thoughts’ (t = 1.92, df = 19, p =
0.07) but these trends were not significant, even before correction for multiple comparisons.
There were no changes in PDI scores post placebo.

Predictors of the mid-term effects of LSD

Regarding baseline predictors of the mid-term effects of LSD, focus was placed on
personality (as measured by the NEO PI-R), depression (measured by the QIDS) and anxiety
(measured by the STAI). A negative correlation was found between baseline agreeableness
and increases in optimism post-LSD (r = -0.56, r 2 = 0.31, p = 0.014) and positive correlations
were found between baseline depression (r = 0.53, r 2 = 0.28, p = 0.024) and anxiety (r = 0.49,
r 2 = 0.24, p = 0.04) and the post- LSD increases in openness. However, none of these survived
correction for multiple comparisons (revised p = 0.05/7 = 0.007).

13

Regarding acute predictors of the mid-term effects to LSD, there was a negative correlation
between acute anxiety (ASC) and post-LSD increases in optimism (r = -0.47, r 2 = 0.22, n =
19, p = 0.04) and positive correlations between impaired/disorganised cognition scores on the
ASC and PSI and post-LSD increases in openness (ASC: r = 0.44, r 2 = 0.2, n = 20, p = 0.049;
PSI: r = 0.52, r 2 = 0.26, n = 20, p = 0.019) but these correlations also failed to survive
correction for multiple comparisons (revised p = 0.5/17 = 0.003).

Discussion

The primary aim of this study was to investigate the acute and mid-term subjective effects of
LSD in a controlled study in order to advance our understanding of its psychological effects.
It was hypothesised that LSD would increase emotional lability and psychosis-like symptoms
acutely, and increase optimism and openness two weeks later, and these hypotheses were
supported by the data.

Focusing first on LSD’s acute effects, scores on the PSI reinforce the view that LSD is a
potent psychotomimetic. Previous studies have examined PSI scores alter sleep deprivation
(Petrovsky et al, 2014), dreaming (Mason and Wakerley, 2012), cannabis (Mason et al,
2008), THC (Stokes et aL, 2009) and ketamine intoxication (Mason et aL, 2008). Although
both dreaming and sleep deprivation elevated subscales of the PSI, only the aforementioned
drugs produced appreciable psychosis-like symptoms - and still not to the same extent as
LSD in the present study. For example, mean total scores on the PSI were 24 ± 10.9 post-
ketamine infusion (150mg/ml plasma), 15.9 ±11 post- THC (lOmg p.o.) and 33.3 ± 19.5 post-
cannabis-use (smoked, quantity unknown), whereas the mean total PSI score in the present
study was 45.8 ±21. To our knowledge, only cannabis in highly schizotypal individuals
(Mason et al., 2009) has produced PSI scores of an equivalent magnitude to those seen here
with LSD.

14

More generally, relatively strong subjective effects were produced by the present dose of
LSD (re. 75pg, i.v.) as indexed by the AS C. In previous studies, psilocybin (115-350pg/kg
PO), MDMA (1.5-1.7mg/kg PO), and ketamine (6-12pg/kg per min Lv.), produced effects of
a lower magnitude than those seen here (Studems et aL, 2010) but in a recent study with
200pg LSD (p.o.), ASC scores were of a similar magnitude to those observed here (Schmid
et aL, 2014).

With the PSI results in mind, it is worth briefly discussing the relative merits of different drug
models of psychosis. Ketamine is often described as an especially meritorious model of
psychosis because it can induce both positive and negative symptoms (Krystal et aL, 1994).
However, the premise that negative symptoms can be modelled by an acute drug state is
questionable (Carhart- Harris et aL, 2013). For example, it has been argued that negative
symptoms are non-specific for psychosis (Le. they are also prevalent in depression (Kaiser et
aL, 2011, Carhart- Harris etaL, 2013). Positive symptoms are specific for psychosis however
but comparison studies have suggested that these are better modelled by classic psychedelics
than ketamine (Carhart- Harris et aL, 2013, Gouzoulis-Mayfrank et aL, 2005). Others have
shown that THC is a particularly potent inducer of positive psychotic symptoms (Morrison et
aL, 2009). Thus, future studies might compare the psychotomimetic properties of THC,
ketamine and a classic psychedelic in a controlled, cross-over design to systematically assess
the relative value of these different models.

Based on the PSI results, one might infer that participants’ acute LSD experiences were
dominated by unpleasant psychosis-like phenomena; however, this was not the case. Some
volunteers did show frank psychotic phenomena during their LSD experiences (e.g. paranoid
and delusional thinking) but at the group level, positive mood was more common. For
example, scores on the (positively valenced) ‘blissful experience’ dimension of the ASC were
significantly higher than scores on the (negatively valenced) ‘anxiety’ dimension. Consistent
with the view that LSD is more likely to produce positive than negative mood, a separate
study with LSD (200pg, p.o.) reported marked positive mood effects, described as ‘MDMA-
like’ (Schmid et aL, 2014); an important caveat however, is that both the Schmid et aL study

15

and the present one, recruited volunteers with previous experience with psychedelics. Thus,
positive prior experiences and positive regard towards LSD may have biased outcomes.

The findings that optimism and openness are increased 2 weeks after LSD are consistent with
previous findings (MacLean et al., 2011, Griffiths et aL, 2006, Griffiths et al., 2008),
suggesting that improved psychological wellbeing and increased openness are relatively
reliable mid to long-term effects of psychedelics. That there were no increases in psychotic
symptomatology at the 2 week end point is also consistent with reports of preserved or even
improved mental health among populations of people that have used psychedelic dmgs
(Krebs and Johansen, 2013, Hendricks et aL, 2015, Bouso et aL, 2012). These results are also
consistent with previous findings that psychedelics can be useful in treating certain
psychiatric disorders (Grob et aL, 2011, Gasser et aL, 2014, Johnson et al., 2014,

Bogenschutz et al., 2015, Moreno et aL, 2006) as well as the notion that they may have
therapeutic potential in the treatment of mood disorders such as depression (Carhart- Harris et
aL, 2014b). High dispositional optimism is associated with a range of positive health and
socioeconomic outcomes (Carver and Scheier, 2014); thus, the increases in optimism
observed here may be treated as further evidence of the therapeutic potential of psychedelic
drugs.

A limitation of the present study was the relatively short duration of the follow-up period (i.e.
2 weeks); however, longer-term follow-up is planned and two weeks is still relevant when
considering potential therapeutic applications (Zarate et aL, 2006). Another limitation is the
single-blind design; however, the classic double-blind model can feel contrived in the context
of controlled studies with potent psychoactive compounds such as LSD, since the blind is
almost universally ineffective. A single-blind design allowed us to introduce some
uncertainty however, and so was considered better than an entirely open- label design. The
inclusion of an ‘active placebo’ condition might be one way to address the ineffectiveness of
the blind in studies with psychedelics; however, an inert placebo was required in the present
study in order to provide a valid baseline for the neuro imaging contrasts. Related to this, a
final limitation to mention is that the LSD and placebo sessions involved MRI and MEG
scans that each lasted for over 60 minutes. Despite careful screening, preparation and some

16

prior exposure to MRI and/or MEG, brain imaging environments can be particularly
demanding for some individuals, particularly under psychedelics (Studems et aL, 2011), and
this may have contributed to the pronounced PSI scores observed here with LSD. Individuals
are known to be especially sensitive to the environment in which they experience the effects
of a psychedelic (Johnson et aL, 2008), and so this factor should be considered when
evaluating the present study’s findings.

With these caveats entered, it is important to attempt an explanation of how LSD can be both
acutely psychotomimetic and yet psychologically beneficial in the mid to long-term, and
insights from neurobiology may be helpful in this regard. There is a consensus that serotonin
2A receptor (5-HT2aR) agonism is central to the psychopharmacology of psychedelics, e.g.
affinity for the 5-HT2aR correlates strongly with their potencies (Glennon et aL, 1984), and
pre-treatment with a 5-HT2aR antagonist effectively abolishes the classic ‘psychedelic’
effects of psilocybin (VoUenweider et aL, 1998). The principal psychological purpose or
function of the 5-HT2aR is poorly understood; however, it is known that 5-HT2aR stimulation
promotes certain aspects of learning (Romano etaL, 2010, Harvey et aL, 2004, Harvey, 2003)
and cognition (Williams et aL, 2002, King et aL, 1972). Specifically, 5-HT2aR stimulation
enhances the flexibility of cognition (Boulougouris etaL, 2008), which may be related to
reports of enhanced imagination (Carhart- Harris, 2014) and creative thinking (Lrecska et al.,
2012) with psychedelics. Interestingly, an association has been found between positive mood
and flexible, creative thinking (Hirt et al., 2008, Lin et aL, 2014) and this may explain why
both are associated with psychedelics. Indeed, the positive mood effects of psilocybin (a
direct 5-HT2aR agonist) and MDMA (a potent serotonin ‘releaser’) are significantly
attenuated by pre- treatment with the 5-HT2aR antagonist ketanserin (van Wei et aL, 2012,
Kometer et aL, 2012).

Psychedelics also transiently impair certain aspects of cognition however, such as the ability
to focus and concentrate (Umbricht et aL, 2003, VoUenweider et aL, 2007), which is
consistent with the high ratings of impaired cognition in the present study (figure 1 & 2). It is
also important to acknowledge that whUe 5 -HTzaR antagonism attenuates the positive mood
effects of psUocybin (and MDMA), it also attenuates positive psychotic symptoms that can be

17

produced by the drug (Carter et aL, 2007) and 5-HT2 aR agonism has been linked with anxiety
related behaviours in rodents (Weisstaub et aL, 2006).

Thus, 5-HT2 aR stimulation has been associated with both positive and negative facets of the
acute psychedelic state (Le. positive mood but also anxiety and psychotic symptoms), so how
can we reconcile these things with each other? Is there a more fundamental action of
psychedelics that can explain both effects? As reported above, 5-HT2 aR stimulation has been
associated with enhanced cognitive flexibility; inspired by recent neuroimaging findings
(Roseman etaL, 2014, Tagliazucchi et al., 2014, Muthukumaraswamy et al., 2013, Carhart-
Harris et aL, 2012, Petri et aL, 2014), the psychedelic state has been characterised as an
‘entropic’ state in which the mind/brain operates outside of its normal optimal level of order,
in a realm of relative disorder (Carhart- Harris et al., 2014b). It may be that what underlies
both facets of the psychedelic state and can resolve the ‘valence paradox’ therefore, is this
principle of increased cognitive entropy (see figure 4). Accordingly, we predict that
disordered or entropic cognition is a more fundamental characteristic of the psychedelic state
than either positive or negative mood. This is something that could be tested by carrying out a
principal components analysis of the ASC data; we would predict that valence non-specific
items and particularly those related to ‘entropic cognition’ (such as loss of sell/cgo. loss of
ego-boundaries, altered meaning and muddled thinking) would load more heavily onto the
first principal component than valence specific items (such as feeling euphoric or sad). Acute
entropic processes may also be useful predictors of the mid/long- terms effects of
psychedelics, and the (trend-level) relationship between impaired/disordered cognition and
subsequent increases in trait openness might be suggestive of such an association.

18

Time/phase

Neurobiological

conditions/characteristics

Psychological characteristics

D Before
exposure to 1
psychedelic

l *

1 Deficient 5-HT2AR stimulation

L Pessimism neuroticism rigid
thinking / T'

Initial action of
psychedelic

n

Sudden 'blast' of 5-HT2AR
stimulation

Unconstrained/'entropic' cognition
*T\ altered perceptual processing / 1\
emotional lability T, uncertainty T,
6 ego-disintegration / b, muddled
thinking/thought disorder / |\
heightened imagination T\ potential
for paranoia & delusional thinking T

Acute state
produced by 1
psychedelic

1 Unconstrained brain network
dynamics ('entropic brain')

9 Enduring
effects of 1
psychedelic

1 "“Enduring 'loosening' of
T brain network dynamics

1 Optimism I s , Openness / T',

T wellbeing "t"

Figure 4 here (schematic)

Figure 4. The action of psychedelics on the mind and brain: This empirically informed
model illustrates the hypothesised relationship between three different neurobio logical or
physiological states and their psychological counterparts. Specifically, it is predicted that
deficient 5-HT2 aR stimulation has a stultifying influence on cognition, promoting pessimism,
neuroticism and rigid thinking (phase 1). Informed by neuroimaging studies with
psychedelics (Carhart- Harris et aL, 2014a), 5-HT2 aR stimulation is associated with
unconstrained brain network dynamics and the characteristic ‘entropic’ quality of cognition
in the psychedelic state (phase 2). Finally, it is hypothesised that an acute ‘onslaught’ or
‘blast’ of 5-HT2 aR stimulation, via the action of a psychedelic, has a residual influence on
brain network dynamics and associated cognition (phase 3). 5-HT2 aR stimulation is described
as having a ‘loosening’ or ‘lubricating’ influence on cognition and this is hypothesised to be
conducive to improved psychological wellbeing. *The long-term effects of psychedelics on
brain network dynamics have yet to be formally investigated.

19

Before concluding, it is worth considering one final important matter. Hie discussion so far
has focused mainly on the paradoxical acute psychological effects of psychedelics. However,
as the present results have demonstrated, the acute effects of psychedelics can be quite
different to their longer-term effects, and it is arguably the latter that are more clinically
relevant. Previous studies have shown that trait characteristics, such as personality and
outlook, can be significantly altered by psychedelics (MacLean et aL, 2011, Griffiths etaL,
2008). Moreover, associations have been found between certain psychological traits and the
5-HT2aR (Frokjaer et aL, 2008, Meyer et aL, 2003, Ott et aL, 2005, Turecki et aL, 1999,
Bhagwagar etaL, 2006). Specifically, deficient 5-HT2aR stimulation has been linked with
depression-related behaviours such as suicide (Turecki et aL, 1999), dysfunctional or
excessively pessimistic attitudes (Bhagwagar et aL, 2006, Meyer et aL, 2003) and neuroticism
(Frokjaer et aL, 2008). In this context, increased psychological wellbeing (Griffiths et al.,
2008), openness (MacLean etal., 2011), decreased suicidality (Hendricks et al., 2015) and
now increased optimism after a ‘blast’ of 5-HT2aR stimulation may begin to make sense. We
predict that deficient 5-HT2aR stimulation causes cognition to ‘stultify’, whereas 5-HT2aR
stimulation loosens cognition and associated brain dynamics, serving as a metaphorical
‘lubricant’ for the mind and brain. We predict that this loosening effect persists beyond the
acute intoxication phase and can potentially explain the mid to long-term psychological
effects of psychedelics. Recent reports of enduring brain changes with 5-HT2aR stimulation
and psychedelic drug-use (Bouso et al., 2015), as well as speculations on the function of
serotonin in the brain (Branchi, 2011), may herald the beginnings of an understanding of this
important matter. We intend to detail the acute brain effects of LSD in forthcoming
neuroimaging papers and to investigate the long-term psychological and brain effects of
psychedelics in future studies.

In conclusion, this study sought to investigate the paradoxical psychological effects of LSD
in a controlled study in healthy volunteers. LSD produced robust acute psychological effects
that were characterised by psychosis-like symptoms but also positive mood. Mid-term effects
included significant increases in optimism and the personality trait openness and no increases
in delusional thinking. A mechanistic explanation for these findings was proposed based on
the ‘entropic brain’ hypothesis (Carhart- Harris et al., 2014a).

20

References

BECK, A.T., WARD, C. H„ MENDELSON, M„ MOCK,J. & ERBAUGH, J. 1961. An inventoryfor
measuring depression. Arch Gen Psychiatry, 4, 561-71.

BHAGWAGAR, Z., HI NZ, R., TAYLOR, M„ FANCY, S., COWEN, P. & GRASBY, P. 2006. Increased 5-

HT(2A) receptor bind ingin euthymic, medication-free patients recovered from depression: a
positron emission study with [(11)C]MDL100, 907. Am J Psychiatry, 163, 1580-7.

BOGENSCHUTZ, M. P„ FORCEHIMES, A. A., POMMY, J. A., WILCOX, C. E„ BARBOSA, P. & STRASSMAN,
R. J. 2015. Psilocybin-assisted treatmentforalcohol dependence: A proof -of-conce pt study J
Psych opha rma col.

BOULOUGOURIS, V., GLEN NON, J. C. & ROBBINS, T. W. 2008. Dissociable effects of selective 5-HT2A
and 5-HT2C receptor antagonists on serial spatial reversal learning in rats.
Neuropsychopharmacology, 33, 2007-19.

BOUSO, J. C., GONZALEZ, D„ FONDEVILA, S., CUTCHET, M„ FERNANDEZ, X., RIBEIRO BARBOSA, P. C.,
ALCAZAR-CORCOLES, M. A., ARAUJO, W. S., BARBANOJ, M. J., FABREGAS, J. M. & RIBA, J.

2012. Personality, psychopathology, life attitudes and neuropsychological performance
amongritual users of Ayahuasca:a longitudinal study. PLoSOne, 7,e42421.

BOUSO, J. C., PALHANO-FONTES, F„ RODRIGUEZ-FORNELLS, A., RIBEIRO, S., SANCHES, R„ CRIPPA, J.
A., HALLAK, J. E., DE ARAUJO, D. B. & RIBA, J. 2015. Long-term use of psychedelicdrugs is
associated with differences in brain structure and personality in humans. Eur
Neuropsychopharmacol.

BRAN CHI, I. 2011. The double edged sword of neural plasticity: increasing serotonin levels leads to
both greater vulnerability to depression and improved capacity to recover.
Psychoneuroendocrinotogy, 36, 339-51.

CARHART-HARRIS, R„ BRUGGER, S., NUTT, D. & STONE, J. 2013. Psychiatry's nexttop model: cause
for a re-think on drug models of psychosis and other psychiatric disorders. J
Psychopharmacoi, 27, 771-8.

CARHART-HARRIS, R„ KAELEN, M„ WHALLEY, M., BOLSTRIDGE, M„ FEILDING, A., NUTT, D. 2014. LSD
enhances suggestibility in healthy volunteers. Psychopharmacology (Berl).

CARHART-HARRIS, R. L., ERRITZOE, D„ WILLIAMS, T., STONE, J. M„ REED, L. J., COLASANTI, A.,

TYACKE, R. J., LEECH, R., MALIZIA, A. L„ MURPHY, K., HOBDEN, P„ EVANS, J., FEILDING, A.,
WISE, R. G. & NUTT, D. J. 2012. Neural correlates of the psychedelicstate as determined by
fMRI studies with psilocybin. Proc Natl Acad Sci USA, 109, 2138-43.

CARHART-HARRIS, R. L., LEECH, R„ HELLYER, P. J., SHANAHAN, M„ FEILDING, A.,TAGLIAZUCCHI, E„
CHIALVO, D. R. & NUTT, D. 2014a. The entropic brain: a theory of conscious states informed
by neuroimaging research with psychedelicdrugs. Front Hum Neurosci, 8, 20.

CARHART-HARRIS, R. L., LEECH, R„ TAGLIAZUCCHI, E„ HELLYER, P. J., CHIALVO, D. R„ FEILDING, A. &
NUTT, D. 2014b. The entropic brain: Atheoryofconsciousstatesinformed by neuroimaging
research with psychedelicdrugs. Frontiers in Fluman Neuroscience, 8, 20.

CARTER, O.L., HASLER, F„ PETTIGREW, J. D., WALLIS, G. M., LIU, G. B. & VOLLENWEIDER, F.X. 2007.
Psilocybin links binocular rivalry switch rate to attention and subjective arousal levels in
humans. Psychopharmacology (Berl), 195, 415-24.

CARVER, C. S. & SCHEIER, M. F. 2014. Dispositional optimism. Trends Cogn Sci, 18, 293-9.

COHEN, S. & DITMAN, K. S. 1962. Complications associated with lyse rgicacid diethylamide (LSD-25).
JAMA, 181, 161-2.

COSTA, P.T.,JR. & MCCRAE, R. R. 1995. Domains and facets: hierarchical personality assessment
usingthe revised NEOpersonalityinventory.lPersAssess, 64, 21-50.

21

DOLDER, P. C., SCHMID, Y., HASCHKE, M„ RENTSCH, K. M. & LIECHTI, M. E. 2015. Pharmacokinetics
and Concentration-Effect Relationship of Oral LSD in Humans. IntJ Neuropsychopharmacol.

FLOYD, M., SCOGIN, F. & CHAPLIN, W. F. 2004. The Dysfunctional Attitudes Scale: factor structure,
reliability, and validity with older adults. Aging Merit Health, 8, 153-60.

FRECSKA, E., MORE, C. E., VARGHA, A. & LUNA, L. E. 2012. Enhancement of creative expression and
entoptic phenomena as after-effects of repeated ayahuasca ceremonies. J Psychoactive
Drugs, 44, 191-9.

FROKJAER, V. G„ MORTENSEN, E. L., NIELSEN, F. A., HAUGBOL, S., PINBORG, L. H„ ADAMS, K. H„
SVARER, C., HASSELBALCH, S. G., HOLM, S., PAULSON, O. B. & KNUDSEN, G. M. 2008.
Frontolimbic serotonin 2A receptor binding in healthy subjects is associated with personality
riskfactors foraffective disorder. Biol Psychiatry, 63, 569-76.

GASSER, P„ HOLSTEIN, D„ MICHEL, Y., DOBLIN, R„ YAZAR-KLOSINSKI, B„ PASSIE,T. & BRENNEISEN, R.
2014. Safety and Efficacy of Lysergic Acid Diethylamide -Assisted Psychotherapy for Anxiety
Associated With Life-threatening Diseases. J Nerv Ment Dis.

GLAESMER, H„ RIEF, W„ MARTIN, A., MEWES, R„ BRAHLER, E„ ZENGER, M. & HINZ, A. 2012.

Psychometric properties and population -based norms of the Life Orientation Test Revised
(LOT-R). Br J Health Psychol, 17, 432-45.

GLENNON, R. A.,TITELER, M. & MCKENNEY, J. D. 1984. Evidence for5-HT2 involvement in the
mechanism of action of hallucinogenicagents. LifeSci, 35, 2505-11.

GOUZOULIS-MAYFRANK, E„ HEEKEREN, K„ NEUKIRCH, A., STOLL, M„ STOCK, C., OBRADOVIC, M. &
KOVAR, K. A. 2005. Psychological effects of (S) -ketamine and N,N-dimethyltryptamine
(DMT): a double-blind, cross-overstudy in healthy volunteers. Pharmacopsychiatry, 38, 301-
11 .

GRIFFITHS, R„ RICHARDS, W., JOHNSON, M„ MCCANN, U. & JESSE, R. 2008. Mystical -type

experiences occasioned by psilocybin mediate the attribution of personal meaning and
spiritual significance 14 months later. J Psychopharmacol, 22, 621-32.

GRIFFITHS, R. R„ RICHARDS, W. A., MCCANN, U. & JESSE, R. 2006. Psilocybin can occasion mystical -
type experiences havingsubstantial and sustained personal meaningand spiritual
significance. Psychopharmacology (Berl), 187, 268-83; discussion 284-92.

GRINSPOON, L. & BAKALAR, J. B. 1979. Psychedelic drugs reconsidered, NewYork, BasicBooks.

GROB, C. S., DAN FORTH, A. L., CHOPRA, G. S„ HAGERTY, M„ MCKAY, C. R„ HALBERSTADT, A. L. &

GREER, G. R. 2011. Pilot study of psilocybin treatment for anxiety in patients with advanced-
stage cancer. Arch Gen Psychiatry, 68, 71-8.

HARVEY, J. A. 2003. Role of the serotonin 5-HT(2A) receptor in learning. Learn Mem, 10, 355-62.

HARVEY, J. A., QUINN, J. L., LIU, R„ ALOYO, V. J. & ROMANO, A. G. 2004. Selective re modeling of

rabbit frontal cortex: relationship between 5-HT2A receptor density and associative learning.
Psychopharmacology (Berl), 172, 435-42.

HENDRICKS, P. S., THORNE, C. B„ CLARK, C. B„ COOMBS, D. W. & JOHNSON, M. W. 2015. Classic
psychedelic use is associated with reduced psychological distress and suicidality in the
United States adult population. J Psychopharmacol.

HINTZEN, A. & PASSIE,T. 2010. The pharmacology of LSD : a critical review, Oxford, Oxford University
Press.

HIRT, E. R., DEVERS, E. E. & MCCREA, S. M. 2008. I wantto be creative: exploringthe role of hedonic
conti ngency theory i n the positive mood -cognitive flexibility link. J PersSoc Psychol, 94, 214-
30.

HODGUES, W. F. & SPIELBERGER, C. D. 1969. An indicant oftraitor state anxiety? J Consult Clin
Psychol, 33, 430-4.

HOFMANN, A. 1980. LSD : my problem child, N.Y., McGraw-Hill.

JOHNSON, M., RICHARDS, W. & GRIFFITHS, R. 2008. Human hallucinogen research:guidelinesfor
safety. J Psychopharmacol, 22, 603-20.

22

JOHNSON, M. W„ GARCIA-ROMEU, A., COSIMANO, M. P. & GRIFFITHS, R. R. 2014. Pilot study of the
5-HT2AR agonist psilocybin in the treatment of tobacco addiction. J Psychopharmacol, 28,
983-92.

KAELEN, M„ BARRETT, F. S„ ROSEMAN, L., LORENZ, R„ FAMILY, N„ BOLSTRIDGE, M„ CURRAN, H. V.,
FEILDING, A., NUTT, D. J. & CARHART-HARRIS, R. L. 2015. LSD enhancesthe emotional
response to music. Psychopharmacology (Berl), 232, 3607-14.

KAISER, S., HEEKEREN, K. & SIMON, J. J. 2011. The negative symptoms of schizophrenia: category or
continuum? Psychopathology, 44, 345-53.

KING, A. R., MARTIN, I. L. & SEYMOUR, K. A. 1972. Reversal learningfacilitated byasingle injectionof
lysergicacid diethylamide (LSD 25) in the rat. BrJ Pharmacol, 45, 161P-162P.

KOMETER, M., SCHMIDT, A., BACHMANN, R., STUDERUS, E., SEIFRITZ, E. & VOLLENWEIDER, F. X.

2012. Psilocybin biases facial recognition, goal -directed behavior, and mood state toward
positive relativeto negative emotions through differentserotonergicsubreceptors. Biol
Psychiatry, 72, 898-906.

KREBS, T. S. & JOHANSEN, P. O. 2013. Psychedelics and mental health: a population study. PLoSOne,
8, e 63972.

KRYSTAL, J. H., KARPER, L. P., SEIBYL, J. P„ FREEMAN, G. K., DELANEY, R„ BREMNER, J. D., HENINGER,
G. R„ BOWERS, M. B., JR. & CHARNEY, D. S. 1994. Subanestheticeffectsof the
noncompetitive NMDAantagonist, ketamine, in humans. Psychotomimetic, perceptual,
cognitive, and neuroendocrine responses. Arch Gen Psychiatry, 51, 199-214.

LIN, W. L., TSAI, P. H., LIN, H. Y. & CHEN, H. C. 2014. How does emotion influence different creative
performances?The mediating role of cognitive flexibility. Cogn Emot, 28,834-44.

MACLEAN, K. A., JOHNSON, M.W. & GRIFFITHS, R. R. 2011. Mystical experiences occasioned by the
hallucinogen psilocybin lead to increases in the personality domain of openness. J
Psychopharmacol, 25, 1453-61.

MASON, 0., MORGAN, C. J., DHIMAN, S. K„ PATEL, A., PARTI, N. & CURRAN, H. V. 2009. Acute
cannabis use causes increased psychotomimeticexperiences in individuals prone to
psychosis. Psychol Med, 39, 951-6.

MASON, O. & WAKERLEY, D. 2012. The psychotomimeticnatureofdreams:anexperimentalstudy.
Schizophr Res Treatment, 2012, 872307.

MASON, O. J., MORGAN, C. J., STEFANOVIC, A. &CURRAN, H. V. 2008. The psychotomimetic states
inventory (PSI): measuring psychotic-type experiences from ketamine and cannabis.
Schizophr Res, 103, 138-42.

MEYER, J. H., MCMAIN, S„ KENNEDY, S. H„ KORMAN, L., BROWN, G. M„ DASILVA, J. N., WILSON, A.
A., BLAK, T., EYNAN-HARVEY, R., GOULDING, V. S., HOULE, S. & LINKS, P. 2003. Dysfunctional
attitudes and 5-HT2 receptors during depression and self-harm .American Journalof
Psychiatry, 160, 90-9.

MORENO, F. A., WIEGAND, C. B„ TAITANO, E. K. & DELGADO, P. L. 2006. Safety, tolerability, and

efficacy of psilocybin in 9 patients with obsessive -compulsive disorder. J Clin Psychiatry, 67,
1735-40.

MORRISON, P. D„ ZOIS, V., MCKEOWN, D. A., LEE, T. D„ HOLT, D. W„ POWELL, J. F„ KAPUR, S. &
MURRAY, R. M. 2009. The acute effects of syntheticintravenous Delta9-
tetrahydrocannabinol on psychosis, mood and cognitive functioning. Psychol Med, 39, 1607-
16.

MUTHUKUMARASWAMY, S. D„ CARHART-HARRIS, R. L., MORAN, R. J., BROOKES, M. J., WILLIAMS, T.
M„ ERRTIZOE, D„ SESSA, B„ PAPADOPOULOS, A., BOLSTRIDGE, M., SINGH, K. D„ FEILDING,
A., FRISTON, K. J. & NUTT, D. J. 2013. Broadband Cortical Desynchronization Underlies the
Human PsychedelicState. J Neurosci, 33, 15171-15183.

OSMOND, H. & SMYTHIES, J. 1952. Schizophrenias newapproach. J MentSci, 98, 309-15.

23

OTT, U., REUTER, M., HENNIGJ. & VAITL, D. 2005. Evidencefora common biological basisofthe

Absorption trait, hallucinogen effects, and positive symptoms: epistasis between 5-HT2aand
COMT polymorphisms. Am J Med Genet B NeuropsychiatrGenet, 137B, 29-32.

PETERS, E. R., JOSEPH, S. A. & GARETY, P. A. 1999. Measurement of delusional ideation in the normal
population: introducingthe PDI (Petersetal. Delusions Inventory). SchizophrBull, 25, 553-
76.

PETRI, G„ EXPERT, P., TURKHEIMER, F., CARHART-HARRIS, R„ NUTT, D„ HELLYER, P. J. & VACCARINO,
F. 2014. Homological scaffoldsof brainfunctional networks. J RSoc Interface, 11,20140873.

PETROVSKY, N., ETTINGER, U., HILL, A., FRENZEL, L., MEYHOFER, I., WAGNER, M„ BACKHAUS,J.&

KUMARI, V. 2014. Sleep deprivation disrupts pre pulse inhibition and induces psychosis -I ike
symptoms in healthy humans. J Neurosci, 34, 9134-40.

REICH, P. & HEPPS, R. B. 1972. Homicide duringa psychosis induced by LSD. JAMA, 219, 869-71.

ROELOFS, J., MURIS, P.,HUIBERS, M., PEETERS, F. & ARNTZ, A. 2006. On the measurement of

rumination: a psychometric evaluation of the ruminative response scale and the rumination
on sadness scale in undergraduates. .7 Be/iavTTier Exp Psychiatry, 37, 299-313.

ROMANO, A. G„ QUINN, J. L., LI, L„ DAVE, K. D., SCHINDLER, E. A., ALOYO, V. J. & HARVEY, J. A. 2010.
Intrahippocampal LSD accelerates learning and desensitizes the 5-HT(2A) receptor in the
rabbit, Romano et al. Psychopharmacology (Berl), 212,441-8.

ROSEMAN, L., LEECH, R., NUTT, D. J., FEILDING, A. & CARHART-HARRIS, R. L. 2014. The effects of

psilocybin and MDMA on between-networkrestingstate functional connectivity in healthy
volunteers. Frontiers in Human Neuroscience, 8.

RUSH, A. J ., TRI VEDI, M. H„ IBRAHIM, H. M„ CARMODY, T. J., ARNOW, B., KLEIN, D. N„ MARKOWITZ,
J.C., NINAN,P.T„ KORNSTEIN, S., MANBER, R., THASE, M. E„ KOCSISJ.H. & KELLER, M. B.
2003. The 16-Item Quick Inventory of Depressive Symptomatology (QIDS), clinician rating
(QIDS-C), and self-report (QIDS-SR): a psychometric evaluation in patients with chronic major
depression. Biol Psychiatry, 54, 573-83.

SCHEIER, M. F., CARVER, C. S. & BRIDGES, M. W. 1994. Distinguishingoptimismfrom neuroticism

(and trait anxiety, self-mastery, and self-esteem): a reevaluation of the Life Orientation Test.
J Pers Soc Psychol, 67, 1063-78.

SCHMID, Y., ENZLER, F„ GASSER, P„ GROUZMANN, E„ PRELLER, K. H„ VOLLENWEIDER, F. X.,
BRENNEISEN,R„ MULLER, F., BORGWARDT, S. & LIECHTI, M. E. 2014. Acute Effects of
LysergicAcid Diethylamide in Healthy Subjects. Biol Psychiatry .

STOKES, P. R„ MEHTA, M. A., CURRAN, H. V., BREEN, G. & GRASBY, P. M. 2009. Can recreational

doses of THC produce significant dopamine release in the human striatum? Neuroimage, 48,
186-90.

STUDERUS, E„ GAMMA, A. & VOLLENWEIDER, F. X. 2010. Psychometricevaluation of the alte red
states of consciousness rating scale (OAV). PLoSOne, 5, e 12412.

STUDERUS, E„ KOMETER, M„ HASLER, F. & VOLLENWEIDER, F. X. 2011. Acute, subacute and long-
term subjective effects of psilocybin in healthy humans: a pooled analysis of experimental
studies .J Psychopharmacol, 25, 1434-52.

TAGLIAZUCCHI, E„ CARHART-HARRIS, R„ LEECH, R„ NUTT, D. & CHIALVO, D. R. 2014. Enhanced
repertoire of brain dynamical states duringthe psychedelicexperience. arXiv preprint
arXiv:1405. 6466.

TELLEGEN, A. & ATKINSON, G. 1974. Opennesstoabsorbingand self-alteringexperiences

("absorption"), a trait related to hypnotic susceptibility. JAbnorm Psychol, 83, 268-77.

TURECKI, G„ BRIERE, R„ DEWAR, K„ ANTON ETTI, T., LESAGE, A. D., SEGUIN, M„ CHAWKY, N„

VANIER, C„ ALDA, M., JOOBER, R., BENKELFAT, C. & ROULEAU, G. A. 1999. Prediction of level
of serotonin 2A receptorbinding by serotonin receptor 2A genetic variation in postmortem
brain samples from subjects who did or did not commit suicide. AmJ Psychiatry, 156, 1456-
8 .

24

UMBRICHT, D.,VOLLENWEIDER, F.X., SCHMID, L., GRUBEL, C., SKRABO, A., HUBER, T. & KOLLER, R.
2003. Effects of the 5-HT2A agonist psilocybin on mismatch negativity generation and AX -
continuous performance task: implications for the neuropharmacology of cognitive deficits
in schizophrenia. Neuropsychopharmacology, 28, 170-81.

VAN WEL,J. H., KUYPERS, K. P„ THEUNISSEN, E. L, BOSKER,W. M., BAKKER, K. & RAMAEKERS, J. G.

2012. Effects of acute MDMA intoxication on mood and impulsivity: role of the 5-HT2 and 5-
HT1 receptors. PLoSOne, 7, e40187.

VOLLENWEIDER, F. X., CSOMOR, P. A., KNAPPE, B„ GEYER, M. A. & QUEDNOW, B. B. 2007. The
effects of the preferential 5-HT2A agonist psilocybin on pre pulse inhibition of startle in
healthy human volunteers depend on interstimulus interval. Neuropsychopharmacology, 32,
1876-87.

VOLLENWEIDER, F. X., VOLLENWEIDER-SCHERPENHUYZEN, M. F„ BABLER, A., VOGEL, H. & HELL, D.
1998. Psilocybin induces schizophrenia -like psychosis in humans via a serotonin -2 agonist
action. Neuroreport, 9, 3897-902.

WEISSTAUB, N . V., ZHOU, M„ LIRA, A., LAMBE, E., GONZALEZ-MAESO, J„ HORNUNG, J. P„ SIBILLE, E.,
UNDERWOOD, M., ITOHARA, S„ DAUER, W. T., ANSORGE, M. S., MORELLI, E., MANN, J. J.,
TOTH, M., AGHAJANIAN, G., SEALFON, S. C., HEN, R. & GINGRICH, J. A. 2006. Cortical 5-HT2A
receptorsignaling modulates anxiety -I ike behaviors in mice. Science, 313, 536-40.

WILLIAMS, G. V., RAO, S. G. & GOLDMAN-RAKIC, P. S. 2002. The physiological role of 5-HT2A
receptors in working memory. J Neurosci, 22, 2843-54.

ZARATE, C. A., JR., SINGH, J. B„ CARLSON, P. J., BRUTSCHE, N. E„ AMELI, R., LUCKENBAUGH, D. A.,
CHARNEY, D. S. & MANJI, H. K. 2006. A randomized trial of an N-methyl-D-aspartate
antagonist in treatment-resistant majordepressi on. Arch Gen Psychiatry, 63, 856-64.

25